Directdebit: Make select_directdebit_invoices() only return invoices whose amount is above zero, because directdebiting invoices whose amount is zero doesn't make sense and whose amount is below zero is illegal. Also, make create_directdebit_transaction() die() on invoices whose amount is below zero for the same reason. Add tests to verify this.
Remove porting_state column from the SIM table, it gives no additional information.
Liminfra-shell: don't quit when the user interrupts during adding a directdebit authorization
Limesco shell: Trim all whitespace off both sides of an interactive question answer.
cdr-import: when a day failed, give an error about it after running. Adds method get_cdr_import_errors including a test.